This invention relates to wire bondable and solderable ultra-thin metal surface finishes for packaging electronic devices such as integrated circuits.
Integrated circuit (IC) devices find wide use in products including consumer electronics, household appliances, computers, automobiles, telecommunications, robotics and military equipment. These devices typically comprise an integrated circuit unit (IC unit), a lead frame and a protective enclosure. The IC unit encompasses one or more integrated circuit chips on a plastic or ceramic support base.
The lead frame electrically interconnects the IC unit to circuitry external of the IC device. The lead frame includes a plurality of conductive leads (or lead fingers) defining a central area in which the IC unit is mounted. It also typically includes a mounting paddle and a plurality of separate lead members extending away from a location adjacent to the paddle. In those instances where the paddle is absent, the leads are formed so that the ends of the leads are overlapping the periphery of the IC unit or the ends of the leads are positioned in an abutting or spaced position with the IC unit.
There are several attachment techniques by which the leadframe connects the IC devices in a package. These include die-attachment with solder or conductive adhesive, wire bonding and encapsulation. In all instances attachment requires a particular quality of the leadframe surface. The surface must be oxide free and ready for interaction with other components such as solder, conductive adhesive and gold or aluminum wire. The surface finish of the leadframe plays an important role in providing reliable, reproducible attachment. The external leads should be solderable for connection to external devices such as printed circuit boards.
Lead frames are typically copper alloy electroplated with a layer of nickel. The nickel plating serves as a barrier to diffusion of copper and to the formation of reactive copper products, such as copper oxide and sulfides. Unfortunately, nickel layers in thickness of less than 400 microinches (10.2 micrometers) contains pores through which migration and diffusion of copper to the surface of the lead frame takes place. However, nickel layers having thickness of greater than 400 microinches tend to crack when the leads are eventually bent.
The presence of nickel and nickel products, such as nickel oxide, at the surface of the leads is of concern from the wirebonding and solderability standpoint. Nickel products, such as nickel oxides, interfere with wirebonding and soldering and they are difficult to remove with conventional acidic cleaning.
An attempt to eliminate or at least reduce effects of diffusion of copper through a less than 400 microinches thick nickel layer was made by depositing a thin layer of palladium or palladium/nickel alloy on top of the nickel layer. (See European Patent Application No. 0 250 146 published Dec. 23, 1987). However, copper corrosion products, including oxides, sulfides and other reaction products of copper, continued to appear on the lead frame, discoloring the surface of the lead frame and degrading its wirebonding and soldering ability. A further attempt to overcome these shortcomings was made by plating the copper base with a plurality of layers including, in an ascending order from the copper base, a 5 microinch (127 nanometers) thick nickel strike layer, a 3 microinch (76 nanometers) thick palladium/nickel alloy layer, a nickel layer and a palladium layer. The nickel strike layer and the palladium/nickel alloy layer were intended to act as a barrier to copper ion migration to the surface of the lead frame so as to permit the use of a thinner (less than 400 microinches) nickel layer. (See European Patent Application No. 0 335 608 published Oct. 4, 1989). However, this combination of layers also did not lead to a product which could withstand the effects of processing steps required in the fabrication of the encapsulated devices.
U.S. Pat. No. 5,360,991 issued on Nov. 1, 1994 to J. A. Abys et al., describes a lead frame comprising a base metal, a layer of nickel on the base metal, and a protective composite of metal layers on the nickel. The composite includes, in succession from the nickel layer, a layer of palladium strike or soft gold strike, a layer of palladium-nickel alloy, a layer of palladium and a layer of gold. The various layers are in thickness sufficient to effectively prevent migration of copper and nickel and their respective corrosion products to the surface of the lead frame, depending on the processing and use conditions especially after being subjected to processing thermal conditions exceeding 250xc2x0 C. Typically the composite is deposited in a total thickness ranging from 10 to 300 microinches (2540-76,200 xc3x85) with gold layer being 1 to 100 microinches thick (254-25,400 xc3x85).
U.S. Pat. No. 5,675,177 issued on Oct. 7, 1997 to J. A. Abys et al., describes a metal lead frame comprising a nickel-plated base metal, and a composite of metal layers comprising, in succession from the nickel, 0.5 to 3.5 microinches (127-889 xc3x85) of palladium or gold strike, 0.5 to 5 microinches (127-1270 xc3x85) of palladium-nickel alloy, 0.5 to 5 microinches (127-1270 xc3x85) of palladium and 0 to 1 microinches of (0-254 xc3x85) gold.
While the finish of the aforementioned patents provide good wirebonding performance and solderability, it would be advantageous to reduce the cost of finishing by finding an even thinner finish which can still meet the wirebonding and solderability requirements.
In accordance with the invention, a packaged electronic device comprises at least one electronic device and leads sealed within a protective package. The leads comprise a conductive metal substrate having a composite metal finish with a total thickness of 1000 xc3x85 or less. The finish comprises, in succession from the substrate, 25-750 xc3x85 of palladium alloy and 5-250 xc3x85 of wirebondable and solderable material such as gold or silver or palladium. The substrate is advantageously nickel-plated copper alloy or Fexe2x80x94Ni alloy. The content of palladium in the palladium alloy coating can range from 10-95 weight percent. This finish meets requirements of wirebonding and solderability at a thickness surprisingly lower than previously used packaging finishes.